The present invention relates to devices for stabilizing platforms on which precision processes are conducted. Specifically, a linear actuator in a feedback control system is described which will cancel disturbances added to the platform by operations carried out on the platform, as well as disturbances induced on the platform from external sources.
A motion stabilized platform is necessary to conduct different types of processes. For example, precision lithography is conducted on isolated platforms so that precision resolution may be obtained. In microlithography, used in the semiconductor fabrication art, the X-ray lithography technique is capable of defining features down to 0.25 micron lengths. These processes are carried out on a platform which is usually isolated from external vibrations and influences via air bearings to preserve the resolution of circuit elements created from a mask exposed to X-rays for defining the contours of an integrated circuit.
The growing of semiconductor substrate wafers in space is another area where a stabilized platform could be used. A large size and flawless quality of the wafer requires a vibration-free environment during the crystal growing process.
Other applications for a stabilized platform can include a medical operating table. To control the precise nature of surgical incisions and other procedures, the extraneous motion imparted to the operating table-platform must be minimal. Further, there is an interest in developing isolated platforms for conducting microgravity material processing experiments in space. It is contemplated that in the future, space shuttle missions will conduct such experiments on a floating platform.
Although techniques are available for isolating a platform from external disturbances and vibrations present in the environment (such as air bearing supports), the processes carried out on the isolated platform are themselves a source of self-induced platform forces. Any mechanical motion which takes place on an isolated platform will induce vibration to the platform, thus reducing the precision with which such processes are carried out. Further, the very process of supporting the platform, such as through air bearings, can induce vibrations in the platform. The platforms generally include some tethering i.e., external connections to the platform which, in a tensioned condition, tend to induce vibrations in the platform.
These precision processes in general require that there be stabilization of the platform such that residual vibration has a spectral density below ##EQU1## up to a 10 Hz bandwidth. After a 10 Hz bandwidth, the vibration spectral density is permitted to increase at a 40 dB/Decade rate, without any significant deterioration in the processes being carried out on the isolated platform. This level of precision requires that the platform include some type of stabilization system which opposes these induced vibration forces.
One way of effecting stabilization of a platform is illustrated in U.S. Pat. No. 4,897,582. This system employs a linear actuator which is commutated to produce forces for opposing forces induced on the platform surface. The system actuator requires a commutator and complex switching circuits to produce the damping forces on the platform. The present invention is directed to these types of stabilization systems.